The processing of staple fibers include several steps all directed toward the orientation and maintenance of the fibers in eveness and in parallel relation to one another. Eveness is a term used to define the desired uniformity of fiber diameter without thick and thin places along its length.
More specifically, the processing of staple fibers includes carding clumps or tufts of cotton or synthetic staple. The carding action separates these clumps or tufts into their individual fiber elements, thereby exposing and removing bits of foreign matter enclosed by the unopened fiber aggregates and feeds the cleaned disentangled fibers through the tapered hole or trumpet to define a continuous untwisted strand of fibers known as sliver. The compression of the fibers at the trumpet and calendar rolls of the carding machine causes fibers to be loosely held together to form the sliver. The size of sliver is expressed as the average weight in grains per yard of length, there being seven thousand grains in one pound. The normal range of sliver weights is from 40 to 70 grains per yard.
The card sliver may be delivered to a draw frame which progressively passes or slides fibers by each other, causing a reduction in size of the strand, but not breaking its continuity. The action is obtained by using several pairs of rolls running at different speeds. The purpose of all roller drawing is to straighten the fibers being treated and to reduce the size of the strand which they compose. The straightening is important because it arranges the fibers more nearly parallel to each other and to the direction of the strand. When the fibers are well straightened, the arrangement helps in producing uniform, strong, and smooth yarn.
The placement of the rolls must be adjusted to suit the length of the fibers being handled. Although the fibers of any given cotton are not uniform in length, the drawing rolls must control as many of the fibers as possible. This means that there are fibers longer and shorter than those for which the rolls are set, and those longer and shorter fibers will not be well controlled. The imperfect control of fibers in the drawing leads to uneveness in the strand, producing irregularities or thick and thin places which contribute to uneveness in the finished product. Conversely, the more perfect control of fibers in the drawing leads to eveness of the strand which contributes to the desired uniformity of the finished product.
The sliver is reduced in diameter by successive drafting and is processed on a spinning frame to provide additional drafting and twisting to produce the desired yarn.
Prior attempts have been made to control the fibers on the draw frame, roving frame and spinning frame including the use on draw frames and roller drafters of a saw-tooth roll with teeth extending at a positive angle to pull the fibers and separate them into spaces between the teeth. The pulling of the fibers by the positively angled saw-teeth is objectionable because it disorients the previous parallelization of the fibers. Balloon rolls have been used and they provide effective tension and maintenance of the yarns. The same is true of the prior art aprons comprising endless rubber belts extending around a special cradle on a spinning frame, known as the Casablanca system, and so arranged that the fibers on the spinning frame are fed between the aprons at their nip. The aprons satisfactorily control the fibers but are objectionable because of the need for frequent replacement. Another objection of the Casablanca system is its expense. The Casablanca system includes special stands for holding the front and back rolls on the spinning frame and the middle bottom roll is of special design with narrow bosses, each of which carries a short endless apron. The middle top rolls, the bosses of which are smaller in diameter than regular top rolls, are also narrow and each boss carries a short, endless apron like those on the bottom roll. It has been estimated that the cost of these special rolls and aprons amounts to 70% of the cost of a conventional spinning frame.